The present invention relates to a thin film electroluminescent element (which is called a thin film EL element in the following description) and a large picture polychromatic display apparatus using this thin film EL element.
The thin film EL element utilizes a luminous phenomenon caused when a strong electric field is applied to a fluorescent material, and has been applied to a large picture flat panel display having excellent visuality.
In general, ITO(Indium Tin Oxide) is used as a material of a transparent electrode in the EL element and is a mixture of tin oxide and indium oxide. However, the heat-resisting property of the ITO is bad and it is necessary to heat a substrate to a high temperature when alkaline earth chalcogenide is used as a luminous layer in the EL element. Therefore, there is a case in which the resistance of the ITO is increased or the ITO is decomposed. To solve this problem, there is a trial using zinc oxide having a heat-resisting property better than that of the ITO. However, such a material having an excellent reliability for a long period is not obtained. The resistance of the transparent electrode is increased as the length of the electrode is increased so that this problem is especially serious in the large picture display.
In the polychromatic thin film EL element of a stacking type, it is necessary to dispose a plurality of transparent electrodes and a thermal hysteresis is repeated in every stacking operation of luminous and insulating layers. Accordingly, a high thermal stability is needed in the transparent electrodes.
The large picture display apparatus conventionally using a light-emitting diode (LED) as a picture element is practically used. However, the display apparatus using the LED as a picture element has the disadvantages that the element is relatively expensive and it is difficult to position the element, thereby increasing the manufacturing cost of the apparatus. Further, it is difficult to provide a full color since no blue luminescence is practically used. In addition, there are some problems about cost when the large picture polychromatic display apparatus is constructed by a plasma display, a liquid crystal display element, etc. as a picture element. On the other hand, there is a possibility that the thin film EL element is used as the picture element of the large picture polychromatic display apparatus. However, the element having a structure suitable for such an object is not conventionally considered.